Get list item where sublist can be null

Question

Hi I have a situation where I have a Car, and Car has many customizations and customizations possibly have many versions.

I am trying to get the car customization if it has a version that versionName contains "xyz".

var customizationToBeUpdated = car.Customizations.FirstOrDefault(pv =>
                    pv.Versions.Any(ver =>
                        ver.Name.Contains("xyz")));

This code above works when it finds a version that matches, but if it doesn't find throws an exception. Any suggestion on how to deal in the case is null?

Exception: System.NullReferenceException: Object reference not set to an instance of an object.

Cheers

it is not null. Has an Item. @ZoharPeled. If I explicitly looking for the name of it, it is all good. — CidaoPapito, Jul 19 '20 at 12:41
Also "car.Customisations.Where(pv => pv.Versions.Any());" Works fine. — CidaoPapito, Jul 19 '20 at 12:46
Than somewhere you have a version where it's `Name` is null. — Zohar Peled, Jul 19 '20 at 12:47
because `Null.Contains("xyz")` will throw a `NullReferenceException`.... — Zohar Peled, Jul 19 '20 at 12:53
Does this answer your question? [What is a NullReferenceException, and how do I fix it?](https://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Zohar Peled, Jul 19 '20 at 12:54

score 0 · Accepted Answer · answered Jul 19 '20 at 12:56

A null safe code would be using the null conditional (?.) operator with the null coalescing (??) operator.
Something like this:

var customizationToBeUpdated = car.Customizations.FirstOrDefault(pv =>
                    pv.Versions?.Any(ver =>
                        ver.Name?.Contains("xyz") ?? false) ?? false);

Basically, pv.Versions?.Any will only execute Any if Versions is not null, but if it is null, it will simply return null, same as Name?.Contains will only execute Contains is Name is not null - hence the usage of the ?? false in both conditions.

score 0 · Answer 2 · answered Jul 22 '20 at 07:17

Requirement

You wrote:

I am trying to get the car customization if it has a version that versionName contains "xyz".

"The" car Customization? Does the car have exactly one Customization with this VersionName?

Car A has three Customizations: K, L, M
K has one Version: "1.0"
L has two Versions: "1.0" and "2.0"
M has three Versions: "1.0" "1.1" "2.0"

Give me "the" Customization of Car A that has a version "1.0".

Classes

Alas you forgot to give us your classes. If you've followed the entity framework code first conventions you'll have classes like this:

class Car
{
    public int Id {get; set;}
    ... // other properties

    // Every Car has zero or more Customizations (one-to-many)
    public virtual ICollection<Customization> Customizations {get; set;}
}

class Customization
{
    public int Id {get; set;}
    ... // other properties

    // Every Customization belongs to exactly one Car, using foreign key
    public int CarId {get; set;}
    public virtual Car Car {get; set;}

    // Every Customization has zero or more Versions (one-to-many)
    public virtual ICollection<Version> Versions {get; set;}
}

class Version
{
    public int Id {get; set;}
    public string Name {get; set;}
    ... // other properties

    // Every Version belongs to exactly one Customization, using foreign key
    public int CustomizationId {get; set;}
    public virtual Customization Customization {get; set;}
}

This is all that entity framework needs to detect the one-to-many relations.

In entity framework the columns of the tables are represented using non-virtual properties; the virtual properties represent the relations between the tables (one-to-many, many-to-many, ...)

Hence a foreign key is non-virtual; the object that the foreign key refers to is virtual.

Back to your question

Requirement: Given a car and a versionName, give me all Customizations of this Car that has at least one Version with a Name equal to this VersionName

Car car = ...
string versionName = ...

You can start at Customizations or at Versions:

Start at Customizations: Give me the Customizations of my Car that has at least one Version with a Name equal to versionName:

var result = dbContext.Customizations
    .Where(customization => 
        // it is one of the customization of my car:
        customization.CarId == car.Id

        // and this customization has at least one Version with versionName:
        customization.Versions.Where(version => version.Name == versionName))
                              .Any());

You can also start at Versions: From every Version that has a Name equal to versionName and a Customization of my car, give me some properties of the Customization:

var result = dbContext.Versions

    // keep only the Versions that have the correct VersionName
    // and that is a Customization of my car:
    .Where(version => version.Name == versionName
                   && version.Customization.CarId == car.Id)

    // from the remaining Versions Select the properties that you plan to use
    .Select(version => new
    {
         // Select the Customization properties that you want:
         Id = version.Customization.Id,
         CustomizationCode = version.Customization.Code,
         ...
    });

Get list item where sublist can be null

2 Answers2

Requirement

Classes

Back to your question